This invention is in the field of machine tools, and more particularly, to automatic tool changers for use with milling machines.
Conventional milling machines generally include a workpiece support table which may be selectively translated along first (X) and second (Y) axes in a plane, and a cutting element which is rotated about and selectively translated along a third (Z) axis. Typically, a base member supports a saddle adapted for translational motion along the X axis. The saddle in turn supports a workpiece table adapted for translational motion with coupled to the saddle along the Y axis. A column is to the base and supports a head which may be selectively driven in a direction parallel to the Z axis. The head generally includes a spindle and associated drive which is adapted for holding a cutting element, or tool, and rotating that tool about the Z axis at a predetermined angular velocity. In alternate forms, the workpiece support table may be rotatable about an axis parallel to the Z axis.
It is well known to provide a computer-based controller for such a milling machine to permit a sequence of programmed motions of the workpiece table and spindle to accomplish a desired machining operation. There are many occasions when it is desired to provide a succession of such machining operations which may require a different tool to be driven by the spindle during each operation. To accomplish this, automatic tool changers have been developed in the prior art. Such tool changer devices are typically driven by the same computer-based controller which drives the spindle and workpiece table. In general, such devices require a tool magazine which supports a plurality of tools in an associated plurality of carriers arranged in a ring. The carriers in the ring may be selectively indexed by the controller so that a desired tool carrier may be positioned at a tool pick-up location. To load a tool from the pick-up location to the tool-holding element of the spindle, a tool selector is selectively operable by the controller to initially move to the tool pick-up location, grab the tool from its carrier at that location, and then move the tool to the tool-holding element and insert the tool in the tool-holding element. The tool selector is then moved to a position out of the way of the machining operation. To effect a tool change, the controller controls the tool selector to perform the reverse sequence to remove the tool from the spindle and re-insert it in its carrier. After the tool selector is moved out of the way of the magazine, the magazine is then indexed to advance the tool required for the next machining operation to the pick-up location. That tool is then grabbed and inserted into the spindle, as described above. In some prior art systems, the tool selector is "two headed" (i.e. it includes two "grippers", each of which can hold a tool) so that a tool changing operation may be performed in the following sequence. First, the selector selects the new tool from the magazine, placing the selected tool in one of its grippers. Then, the selector travels to the tool holder of the spindle, and first extracts the old tool, placing that tool in the selector's vacant gripper. Then, the selector inserts the new tool in the tool holder, and finally returns the old tool to the magazine. With this configuration, the tool selector only requires a single trip between the magazine and spindle to accomplish a tool change.
In the prior art, the machines are generally arranged so that before each tool changing operation, the head retracts to its fully retracted position along the Z axis and then the tool selector which is coupled at a fixed height to the column then performs its tool changing operation. While this approach is generally efficient in relatively small machines where the quill travel for the head is relatively short, in larger machines which have correspondingly larger quill travel, the requirement for positioning the head to its fully retracted position before changing is both time-consuming and requires relatively large amounts of energy.
In a number of other prior art systems, the full Z-direction retraction step for the head is avoided by mechanically mounting the tool magazine on the head itself. With this configuration, the tool changing operation may be performed with the head minimally retracted from the workpiece, rather than at its fully retracted position. While this approach does eliminate the problems faced by relatively small machines, in that full retraction is not required, the requirement for the head to carry the full magazine in all its motions tends to load the head (requiring larger drive motors and associated mechanizations) since the full magazine must be carried everywhere the head moves. In some machines, this loading is minimized by providing magazines with a relatively small number of tools, although such configurations reduce the versatility of the resultant system.
Accordingly, it is an object of the present invention to provide an improved automatic tool changer for a machine tool.
It is another object to provide an automatic tool changer for a machine tool which permits automatic interchange of tools from a fixed position tool magazine to a spindle at any point along the head travel.